Hexakis(tert-butyldimethylsiloxy)ditungsten and its reaction with ethyne. Hydrogen atom transfer reactions involving bridging ethynyl, ethyne, vinyl and ethylidyne ligands. Crystal structures of [W2(OSiButMe2)6], [W2(OSiButMe2)6(µ-C2H2)(C5H5N)] and [W2(OSiButMe2)5(µ-CCH)

Abstract

In hydrocarbon solvents [W2(OBut)6] and ButMe2Si(OH)(6 equivalents) react at room temperature to give [W2(OSiButMe2)6]1, with the elimination of ButOH. Compound 1 is a red, crystalline, hydrocarbon-soluble, air-sensitive compound and is a member of the family of ethane-like X3MMX3 compounds with W–W 2.324(3), W–O 1.89(1)–1.96(1)A and W–W–O 98–104°. Compound 1 and ethyne react in hexane–pyridine (py)(50:50 v/v) at –10 °C to give a dark brown compound, [W2(OSiButMe2)6(µ-C2H2)(py)]2, which has been fully characterized and shown to contain a central pseudo-tetrahedral W2(µ-C2) unit [W–W 2.653(2), W–C 2.05(2)–2.18(2) and C–C 1.45(3)A]. Compound 2 reacts in hydrocarbon solvents at room temperature to give [W2(OSiButMe2)5(µ-CCH)]3, with the elimination of one equivalent of ButMe2Si(OH) and py. The conversion of 2 to 3 in hydrocarbon solvents is suppressed by added py and the rate of reaction is negligible at and below –45 °C. Compound 3 contains a σ,π-ethynyl ligand and the central W2C2 moiety is planar and unsupported by bridging alkoxides [W–W 2.481(1)A, W–σ-C 1.97(2), W–η2-C 1.96(2) and 2.07(2) and C–C 1.41(3)A]. The conversion of 2 to 3 accounts for ca. 90% of the µ-C2-containing compounds but minor competition reactions yield both a σ,π-vinyl and an ethylidyne species formulated as [W2(OSiButMe2)7(µ-CHCH2)]4 and [W2(OSiButMe2)7(µ-CCH3)]5, respectively. Neither 4 nor 5 has been isolated in a pure form and evidence for the µ-vinyl and µ-ethylidyne complexes rests on 1H and 13C NMR data. From studies of reactions employing labelled ethyne it is shown that the formation of 3 with elimination of ButMe2Si(OH) is irreversible and that the µ-vinyl complex is formed by the reaction between the µ-ethyne complex 2 and ButMe2Si(OH). The µ-vinyl and µ-ethylidyne complexes are formed by independent paths and though they are isomers, being related by a 1,2-H atom shift, they are not interconverted in these reactions. It is proposed that a reactive µ-vinylidene intermediate is responsible for the formation of the ethylidyne ligand: W2(µ-HCCH)→ W2(µ-CCH2); W2(µ-CCH2)+ H → W2(µ-CCH3).